1. Field of the Invention
The present invention relates to a zoom lens device of a two-lens group structure used in a still camera, a video camera, etc.
The present invention also relates to an improvement of a driving device of a varifocal lens having a two-lens group structure in which a focal length of the varifocal lens can be changed by displacing each of front and rear lens groups constituting a zooming optical system along a photographing optical axis.
2. Description of the Related Art
In a general known zoom lens device of a two-lens group structure, the construction of a zooming optical system is simplified and the zooming optical system is made compact.
A zoom lens of a two-lens group structure has various kinds of constructions. There is a zoom lens of a two-lens group structure in which positive and negative lens groups are sequentially arranged from the side of a photographing field. Such a zoom lens can be generally classified into the following three focusing systems in accordance with a difference in focusing operation.
A first focusing system is a front focusing system. In this front focusing system, a positive lens group is moved to control a focusing operation while a negative lens group is stopped in a position in which the negative lens group is located at a zooming time.
A second focusing system is a rear focusing system. In this rear focusing system, the negative lens group is moved to control a focusing operation while the positive lens group is stopped in a position in which the positive lens group is located at the zooming time.
A third focusing system is an entire drawing-out focusing system. In this entire drawing-out focusing system, the positive and negative lens groups are moved to control a focusing operation.
It is important to make a camera compact and increase a zoom ratio so as to improve the portability of a camera and enlarge a photographing condition. It is concretely considered that the camera is made compact and the zoom ratio is increased by reducing a length from a film face within the camera to a front end of a photographing lens.
In the front focusing system and the entire drawing-out focusing system, the positive lens group is further drawn out at the zooming and telescopic times in the ease of a most proximate photographing operation. Accordingly, a length from a film face to the front end of the photographing lens including the length at the focusing time is increased.
In contrast to this, in the rear focusing system, no positive lens group is drawn out. When a distance between a film face and the negative lens group including the distance at the focusing time is equal to zero, the length from the front end of the photographing lens to the film face is set as a minimum limit.
In the front focusing system and the entire drawing-out focusing system, a drawing-out amount of the positive lens group on a telescopic side is reduced when the lengths from the front end of the photographing lens to the film face in the above three focusing systems are equal to each other. Thus, distances between the positive and negative lens groups are increased in the front focusing system and the entire drawing-out focusing system. Accordingly, it is impossible to set a focal length longer than that provided in the rear focusing system.
For example, when the camera is concretely designed without considering a macro photograph, a zoom lens capable of performing a zooming operation in the region of a focal length from 38 to 80 mm fulfills an additional function in which the macro photographing operation can be performed in an entire zooming region. In this case, the size of a zooming region on a wide angle side is reduced to secure a moving amount of the negative lens group on a side of the film face so that only a zooming region of the focal length from 40 to 80 mm is set.
As mentioned above, in the rear focusing system, the zooming region on the wide angle side at a normal photographing time is reduced in size to perform the macro photographing operation of low frequency in use. When the lengths from the front end of the photographing lens to the film face in the above three focusing systems are equal to each other, it is impossible to set a shorter focal length on the wide angle side in comparison with the front focusing system and the entire drawing-out focusing system.
A varifocal lens of a two-lens group structure using the front focusing system and having a lens group moving mechanism with a base frame has the following problems when the varifocal lens is actually manufactured as goods.
In a first problem, the structure of a head portion of the varifocal lens is large-sized since a focusing mechanism portion is disposed in the head portion of the varifocal lens in the front focusing system. Therefore, such a structure of the head portion constitutes a great drawback in structure when the varifocal lens is made compact.
In a second problem, when the focusing mechanism portion is disposed in the head portion of the varifocal lens, it is necessary to protect this focusing mechanism portion from external force applied from the exterior of the focusing mechanism portion. Therefore, the entire length of a lens body tube is increased in the direction of an optical axis so that it is difficult to make the varifocal lens compact.
In a third problem, the varifocal lens is large-sized in the direction of the optical axis in accordance with the moving amounts of front and rear lens groups in the front focusing system.
The third problem is also caused in a simultaneous lens moving system as the entire drawing-out focusing system. In addition to the third problem, the moving amounts of the lens groups are increased and continuously changed at a focusing time in accordance with a realized focal length. Accordingly, it is difficult to make the varifocal lens compact by using the simultaneous lens moving system.
In the general varifocal lens, a zooming region at a normal photographing time is sacrificed for a macroscopic photographing operation having a low frequency in use, or an increase in proximate range for enabling a photographing operation on a wide angle side. Further, it is impossible to increase a zoom ratio of the varifocal lens as much as possible. Accordingly, in accordance with a general idea of the varifocal lens, it is theoretically impossible to increase a field angle at the normal photographing time while a proximate or macroscopic photograph on the wide angle side can be taken.
It is further desirable to set most proximate photographing positions in both photographing modes to positions nearer than most proximate photographing positions provided in the general average varifocal lens.
A mechanical feeding means such as feeding mechanisms of a lead screw system and a gear system has a clearance in engagement between a driving lead screw and a lead nut screwed onto the driving lead screw. Further, there is usually a backlash between driving and driven gears. To realize a high accuracy in operation by using these feeding mechanisms, it is necessary to remove the above engagement clearance and the backlash as much as possible by precisely processing and assembling parts of the feeding mechanisms. In the following description, the engagement clearance and the backlash are generally called a mechanical feeding play.
If the base frame and a frame for holding the rear lens group are moved in a state in which this mechanical feeding play is caused, an error in stopping position is caused in accordance with this feeding play when these two frames are stopped in respective target positions set at an optical designing stage. Accordingly, no positions of the frames can be exactly set and there is a possibility that no predetermined optical performance can be obtained.
However, many operations are required to process and assemble parts of the mechanical feeding means such that the mechanical feeding play can be removed, thereby greatly increasing cost of the varifocal lens.